So i took my car into a shop for a "check-up"

and when they left my car inside the shop they noticed a tiny lil puddle of oil on their shop floor, they said its a very sloww leak, but they said the oil or oil pan gasket or somethin wit gasket and oil and that the gasket was not tightened or loose?
Can anyone describe what the problem is please?And if its sometthing where i can get under the car and tighten or is it more complicated then that? especially since i have the big metal under-ride guard that gets taken off everytime i get an oil change